In certain circumstances, "if __debug__" seems to be evaluating its "else" clause even when -O is not specified.  This can cause very  surprising behavior.

a.zip contains a single file named a/__init__.pyo.  It is the compiled form of the following code:

    if __debug__:
        print 'DEBUG'
    else:
        print 'RELEASE'
        if __debug__ == True:
            raise Exception("this is impossible")

pythonbug.py evaluates this script with the following:

    import sys
    sys.path = ['a.zip']
    import a

When using Windows Python 2.6.2 and 2.6.5, running pythonbug.py produces this output:

    RELEASE
    Traceback (most recent call last):
      File "pythonbug.py", line 3, in <module>
        import a
      File "__init__.py", line 8, in <module>
        raise Exception("this is impossible")
    Exception: this is impossible

When -O is passed, the exception is not raised.

My best guess is that the Python bytecode is optimizing the "if __debug__" test away in the produced .pyo, but does not actually affect the value of __debug__ or check more complex expressions involving __debug__.